This invention relates to a system to monitor changes in acoustic properties during the curing of plastics, and more particularly to a narrowband acoustic technique to make the measurements and the use of a marginal oscillator.
Present technology for curing of fiber-reinforced plastics uses a curing cycle which is empirically determined for the material of interest. Utilization of "smart press" technology to adjust the parameters of the cure, such as temperature and pressure, in response to the state of the material during the cure requires a non-invasive technique for evaluating the degree of cure of the plastic while the cure is in progress. During a cure, plastics undergo large changes in elastic moduli. Therefore, changes in ultrasonic properties such as velocity and attenuation should provide a sensitive and reliable measure of the degree of cure. What is needed is an improved means for determining the velocity and attenuation of ultrasound of a fiber-reinforced plastic during curing which is accurate and yet easy to apply in production work.
As the resin of a fiber-reinforced plastic cures, bonds form between the various polymer chains, and this increasingly complex linkage results in progressive increases in the viscosity, bulk modulus, and shear modulus of the resin matrix and of the overall composite. Longitudinal ultrasonic velocity data demonstrating these changes in epoxy carbon composites have been taken by W. P. Winfree and F. R. Parker, "Measurement of the Degree of Cure in Epoxies with Ultrasonic Velocity", Rev. of Progress in Quantitative NDE, 1985, Vol. 5B, Plenum Press. Previous measurements of velocity have been made using short, broadband pulses. Although this technique is conceptually straightforward, difficulties (due to the highly frequency-dependent attenuation of the composite) in producing pulses which are short enough to allow temporal resolution of distinct echoes from the sample has limited the efficacy of these pulse-echo techniques.
One reference where a marginal oscillator is discussed, in a medical application to measure changes in ultrasonic attenuation due to scattering and absorption of particles in a fluid, is the Ph.D. Thesis of Dennis R. Dietz, Washington University, St. Louis, 1976. Another is M. S. Conradi, J. G. Miller and J. S. Heyman, Rev. Sci. Instrum., 45, 358-360, 1974.